MCQs

According to Chargaff's rules, if C = 30%, then G = 30%. Total C+G = 60%. The remaining 40% is A+T, so A = 20% and T = 20%.

A missense mutation is a single nucleotide change that results in a codon for a different amino acid. Here, arginine is replaced by alanine, which will likely alter the protein's primary structure and potentially its function. A silent mutation codes for the same amino acid.

Enzymes, like all catalysts, speed up the rate of a reaction by providing an alternative pathway with a lower activation energy (Ea). They do not change the overall free energy change (ΔG) or the equilibrium constant of the reaction. Without this rate enhancement, metabolic reactions would be too slow to sustain life.

Inorganic ions, like Mg²⁺, Zn²⁺, or Fe²⁺, that bind loosely to an enzyme and increase its activity are termed activators or inorganic cofactors. A coenzyme is an organic molecule. A prosthetic group is a tightly-bound organic or inorganic molecule.

Cellulose is a linear homopolymer of glucose linked by β-1,4-glycosidic bonds, forming strong microfibrils that are embedded in the plant cell wall matrix. Its primary role is to provide rigidity and structural support to plant cells.

Disulfide bridges (-S-S-) are covalent cross-links formed between cysteine R-groups. They lock the tertiary structure in place. Reducing agents like β-mercaptoethanol break these linkages, which can drastically destabilize the protein's 3D fold, causing unfolding. Detergents and urea primarily disrupt non-covalent interactions.

By binding separate substrates in adjacent binding sites on a single enzyme surface, the enzyme converts a slow, intermolecular, second-order reaction into a much faster, intramolecular, first-order reaction. This drastically increases the probability of productive collisions.

The Anfinsen experiment with ribonuclease showed that the amino acid sequence contains all the information needed for the protein to fold into its correct tertiary structure. Upon removal of a denaturant, the protein refolded spontaneously, proving structure is sequence-determined.

Hydrogenation adds hydrogen atoms across the carbon-carbon double bonds in unsaturated oils, converting them to saturated single bonds. This straightens the fatty acid chains, allowing them to pack more tightly and solidify at room temperature.

This is a classic example of feedback inhibition, a negative feedback loop. The final product binds to an allosteric site on enzyme 4 (often the first committed step enzyme), causing a conformational change that reduces its catalytic activity and shuts down the pathway.

The sugar in RNA is ribose, which has a hydroxyl (-OH) group on the 2' carbon. The sugar in DNA is deoxyribose, which has only a hydrogen atom at the 2' carbon. This single oxygen difference makes RNA chemically more reactive and less stable than DNA.

Molecular chaperones are proteins that assist the non-covalent folding/unfolding and assembly/disassembly of other macromolecular structures. They provide a protected environment for a protein to fold correctly, thereby preventing improper interactions that lead to denaturation and aggregation.

Lysozyme specifically targets the β-1,4 glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in the peptidoglycan layer of bacterial cell walls. This bond cleavage weakens the cell wall and causes bacterial lysis.

The 20 common amino acids all share a common backbone (amino group, α-carbon, carboxyl group) but differ only in their side chain, the R-group. The size, shape, charge, hydrophobicity, and chemical reactivity of the R-group confer the unique properties to each amino acid.

A longer saturated hydrocarbon chain has a greater surface area for van der Waals interactions with neighboring chains, requiring more thermal energy (higher temperature) to disrupt these interactions and melt. Unsaturation, conversely, introduces kinks that lower the melting point.

Except for methionine and tryptophan, all 18 other amino acids are encoded by 2 to 6 synonymous codons. This property is called degeneracy and provides a buffer against the harmful effects of point mutations.

The conformational change of the enzyme upon substrate binding physically distorts the substrate molecule. This "strain" on specific bonds makes them less stable and closer to the transition state, thereby reducing the energy required to break them (the activation energy).

Histones are basic proteins that associate with and neutralize the negative charge of the DNA phosphate backbone. The DNA wraps around an octamer of histone proteins to form a nucleosome, the fundamental unit of chromatin packaging, allowing the long DNA molecule to be compacted.

Zymogens (like pepsinogen to pepsin) are activated by the irreversible hydrolytic cleavage of a portion of their polypeptide chain. This proteolytic cut induces a conformational change that forms the functional active site. This mechanism prevents premature activity in the cell of origin.